zerocopy: Deprecate Ref::cast and introduce the safer Ref::coerce

[udoprog]

This pull request adds Ref::coerce and Ref::try_coerce which are a series of valid conversion.

These methods accomplishes a few things:

• It ensures that common types which have the same size can be coerced to and from each other.
• It defines slice coercions which adjust the size of slices, for example when [T] to [U] conversions are performed. The important aspect here is the pointed-to data.
• Defines a single generic API for performing "valid conversion" for APIs which are interested in that, such as the newly introduced trie which only cares about the byte representation of its strings.

Since coercions might overflow, Ref::try_coerce is available which returns None if metadata coercion overflows.